1. Field of the Invention
The present invention relates to a semiconductor storage device and, more particularly, to a static random access memory.
2. Description of the Prior Art
A static random access memory (hereinafter referred to xe2x80x9cSRAMxe2x80x9d) is useful because it does not require refresh operation. However, the number of elements constituting one memory cell in the SRAM is large to increase an occupied area by the memory cell. For this reason, it is required to reduce a cell area. For example, Japanese Laid-open Patent Publication No.9-270468 (U.S. Pat. No. 5,744,844) and Japanese Laid-open Patent Publication No. 10-178110 (U.S. Pat. No. 5,930,163) illustrate examples of cell layout in which one cell having a length in a word line direction which is larger than a length in a bit line direction. Of the examples, a flat configuration of the SRAM described in Japanese Laid-open Patent Publication No. 10-178110 is shown in FIGS. 16 and 17. FIG. 16 is a plan view related to one memory cell of the SRAM. FIG. 17 is an equivalent circuit diagram corresponding to the memory cell shown in FIG. 16. The length in the bit line direction is shortened to increase the speed, and the layout of an active layer and a gate wiring element has a simple shape to reduce a cell area.
From a viewpoint of decreasing patterning size, a phenomenon (optical proximity) in which a resist pattern on a wafer is distorted becomes conspicuous due to the interference of light in an exposure device. In addition, even in an etching process, pattern distortion is generated by the micro-loading effect after etching. The micro-loading effect is a phenomenon in which an etching rate decreases in a direction of depth when a pattern having a large difference in density. In recent years, in order to minimize these pattern distortions phenomena, a technique for the optical proximity correction (OPC) is developed and used in which a mask pattern is automatically corrected in advance in a photography process.
In general, in order to form a contact by forming a contact hole in a gate wiring element, a cover margin such as a lithographic margin and a machining margin must be set in consideration of blur in photolithography process. For this reason, a portion in which a contact hole should be formed on the gate wiring element must be deformed by increasing the width of the portion by a length corresponding to a cover margin. In addition, since a width must be partially increased to make the width of the gate wiring element fine, decreasing patterning size cannot be achieved easily.
In order to decrease patterning size in consideration of an optical proximity correction (OPC) pattern obtained by the optical proximity correction (OPC) technique, when gate wiring elements are complicatedly arranged, margins for the optical proximity correction must be set in the longitudinal and lateral directions. For this reason, a memory cell area cannot be reduced sufficiently because sufficient decrease in size cannot be achieved, and the margins are factors which hinder decrease in size.
It is an object of the present invention to secure lithographic and machining margins without complicatedly deforming a gate shape in formation of a gate wiring element of a semiconductor storage device, especially, an SRAM.
In accordance with one aspect of the present invention, there is a semiconductor storage device including a memory cell array, a plurality of word lines, a plurality of bit lines, a first gate wiring element, a second gate wiring element, a first connector, and a second connector. The memory cell array has a plurality of memory cells. Each memory cell has first and second driver transistors, first and second load transistors, and first and second access transistors. That is to say, first and second sets each having a driver transistors, a load transistors, and an access transistors are designed in each memory cell within SRAM. The memory cells are two-dimensionally arranged on a semiconductor substrate. The word lines are connected to the memory cells and are arranged in parallel to each other along a first direction. The bit lines are connected to the memory cells and are arranged in parallel to each other along a second direction perpendicular to the first direction. The first gate wiring element composes a gate electrode of the first driver transistor and the first load transistor, and has a rectangular shape having straight line on opposite sides. The second gate wiring element composes a gate electrode of the access transistor, and has a rectangular shape having straight line on opposite sides. That is to say, the first and second gate wiring element have fair lines such as notch-less shape. The first connector connects the first gate wiring element, an active region of the second driver transistor, and an active region of the second load transistor to each other. The second connector connects the second gate wiring element to the word lines.
In another aspect of the present invention, there is a semiconductor storage device including a memory cell array, a plurality of word lines, a plurality of bit lines, a first gate wiring element, and a second gate wiring element. The memory cell array has a plurality of memory cells. Each memory cell has first and second driver transistors, first and second load transistors, and first and second access transistors are two-dimensionally arranged on a semiconductor substrate. The word lines are connected to the memory cells and arranged in parallel to each other along a first direction. The bit lines are connected to the memory cells and arranged in parallel to each other along a second direction perpendicular to the first direction. The first gate wiring element composes a gate electrode of the first driver transistor and the first load transistor. The second gate wiring element is connected to the access transistor.
In the semiconductor storage device according to the present invention, the first gate wiring element and the second gate wiring element have rectangular shapes each having straight line on opposite sides and being free from a notch or a projection, and are linearly laid out. In this manner, since the first and second gate wiring elements can be formed at high accuracy, the characteristics of transistors constituting a memory cells can be stabilized. For this reason, a semiconductor storage device having stable characteristics can be obtained. In the semiconductor storage device, contacts to the respective wiring elements are formed by using local inter connectors (LICs). More specifically, the contacts of the respective gate wiring elements are not formed through via holes directly formed on the gate wiring elements, but the contacts are formed by the local inter connectors (LICs) formed by tungsten damascene. When the local inter connectors (LICs) are used, regular gate wiring elements each having a rectangular shape can be laid out without making a cover margin for contact in formation of the gate wiring elements. In addition, since the first gate wiring elements and the second gate wiring elements are laid out in parallel to each other, in the step of forming gate wiring elements by photolithography process, pattern distortion caused by interference can be suppressed. Therefore, an optical proximity effect in the photolithography process can be suppressed.